The search for alternative means of producing electrical energy has created significant interest in economical manufacture of solar cells Such devices should efficiently collect and convert incident light to electrical energy, should have relatively long lives, and should remain reliable through their useful lives.
Thin film, polycrystalline, heterojunction solar cells are well known and have been the subject to numerous publications, such as U.S. Reissue 31,968 to Mickelsen and Chen, which is incorporated by reference. Generally, that patent discloses adjusting the copper/indium ratio in the chalcopyrite CuInSe.sub.2 portion of a CdS/CuInSe.sub.2 heterojunction to obtain improved conversion efficiencies for the cell.
In U.S. Pat. No. 4,523,051 Mickelsen and Chen disclosed a relatively inexpensive method of making a mixed binary thin film having improved properties over materials made with previous processes. Specifically, they disclosed a method for forming n-type Zn.sub.x Cd.sub.l-x S for a ZnCdS/CuInSe.sub.2 heterojunction solar cell. The process used individual sources for ZnS and CdS of the mixed binary compound and reduced the mean free path of the evaporated elemental particles. The general method of U.S. Pat. No. 4,523,051 is applicable to the present invention and its disclosure is incorporated by reference.
U.S. Pat. No. 4,684,761 to Devaney (incorporated by reference) discloses a CdZnS/CuInSe.sub.2 heterojunction solar cell made by controlling particular evaporation rates during deposition of a CuInSe.sub.2 film to achieve compositions providing heterojunction cells of high efficiency.